Migrating eagles stopping in the Sahara Desert: Aberrant behaviour or foraging strategy?

The Sahara Desert is one of the major ecological barriers of the Palearctic-Africa bird migration system. We analysed the behaviour of GPS-tagged booted eagles (Aquila pennata) breeding in Spain and wintering in Tropical Africa, focusing on the Sahara crossing. We searched for anomalous track segments recorded during the desert crossing, finding that non-directional and/or slow movements occur in 35% of the migratory journeys and last up to 31 days (average: 5.9). The analyses show that this behaviour is not triggered by adverse weather conditions, and is shown especially by females during spring. The length of the stopping periods affects the arrival date to the breeding grounds only in males, which means that females buffer the potential delay with an earlier migration start, suggesting that this behaviour is followed deliberately. These wandering movements occur in hyper-arid areas without vegetation cover. A similar behaviour has not been previously recorded in any other trans-Saharan raptor and is probably related to the ability of booted eagles to take advantage of food resources unexploited by other predators, such as migratory songbirds stopping in the desert during the day, and migratory locusts. If confirmed by further research involving higher resolution tracking and field observations, our findings may show a new way of combining foraging and migration, without a well-defined directed trajectory, nor stopping in a restricted spot, but instead wandering over wide areas.